Electrolytic cell cathode bottom with vertically inserted current conductor



. WRIGGE s ATH Sept. 30. 1969 W LL C INSERTED CUR ELECTROLYTIC CE 4Sheets-Sheet 1 Filed Oct. 27, 1954 FIG,

M II M II INVENTORS Friar/d BY 5m: f Marv MM /bf' ATTORNEY F. w. WRIGGEETAL 3.470,083 ELECTROLYTIC CELL CATHODE BOTTOM WITH VERTICALLY INSERTEDCURRENT CONDUCTORS 4 Sheets-Sheet 2 Sept. 30, 1969 Filed Oct. 27, 1964pt. 30. 1969 F. W.WRIGGE ETAL 3,470,083

ELECTROLYTIC CELL CATHODE BOTTOM WITH VERTICALLY INSERTED CURRENTCONDUCTORS Filed Oct. 27, 1964 4 Sheets-Sheet 3 Sept. 30. 1969 F. w.wmsss ETAL ELECTROLYTIC CELL CATHODE BOTTOM WITH VERTICALLY INSERTEDCURRENT CONDUCTORS Filed Oct. 27, 1964 4 Sheets-Sheet 4 INVENTOR 3United States Patent 3,470,083 ELECTROLYTIC CELL CATHODE BOTTOM WITHVERTICALLY INSERTED CURRENT CONDUCTOR Friedrich Wilhelm Wrigge, BadGodesberg, and Ernst Weckesser, Grevenbroich, Germany, assignors toVereinigte Aluminium-Werke Aktiengesellschaft, Bonn, Germany Filed Oct.27, 1964, Ser. No. 406,841 Claims priority, application Germany, Nov.22, 1963, V 24,909, Patent 1,187,809 Int. Cl. C22d 3/02 US. Cl. 204-243Claims The present invention relates to an electrolytic cell and, moreparticularly, to an electrolytic cell for recovery of a metal from amolten electrolyte containing the same, particularly for the recovery ofaluminum in a cell having a bottom-forming cathode of carbonaceousmaterial.

The structure of the cathode has always found great attention in thearrangements for electrolytic recovery of metals from moltenelectrolyte, particularly for the recovery of aluminum because theuseful life span of the electrolytic cell depends on the durability ofthe cathode.

In the beginning of the aluminum recovery by means of electrolyticcells, the cathode-forming carbon bottom of the electrolytic cell restedon a cast iron plate and current conduction to the cathode was improvedby expending bolts which were screwed into the carbon plate, dovetailribs or similar devices which extended from the cast iron plate into thecarbon electrode. However, with progressively increasing size of thecells, this cathode structure was found to be less and lesssatisfactory. It was re placed with bottom forming cathodes of preburnedcarbon blocks or stamped carbonaceous masses whereby, in the case ofpreburned carbon blocks, the joints were between the individual blockswere filled with stamped carbonaceous mass. Current was introduced bymeans of steel rods of various dimensions and arrangements.

The large cells which are presently generally used for the electrolyticrecovery of aluminum generally include a bottom forming cathodeconsisting of preburned rectangular carbon blocks in the lower face ofwhich, in preformed grooves, iron rails are located and the remainder ofthe groove is then filled with stamped or compressed carbonaceous mass.The joints between the individual carbon blocks are also filled withcarbonaceous mass which is either stamped or poured into the joints.

Upon starting operation of a thus formed electrolytic cell, the cathodehas to be heated as evenly as possible so as to be burned into a unitarycarbon block. Only after such unitary carbon block has been formed canthe cell be placed into operation, i.e. molten electrolyte poured ontothe bottom forming carbon block cathode. It is a prerequisite for theburning of the cathode that by coating of the stamped carbonaceous massa highly conductive connection is formed between the carbon block andthe current conducting iron rails so that the transfer resistance willbe as small as possible. On the other hand, due to the firm adherence ofthe carbon to the iron rails and to the very different heat expansion ofthe two materials, i.e. the carbon and the iron, considerable mechanicalstresses will occur which lead to the formation of transversal cracks inthe carbon blocks.

It has been tried to counteract this mechanical destruction of thecathode in various manners. The shape and position of the iron rails wasvaried in all possible directions. However, in the case of large cellsrequiring current of for instance 100,000 amperes, the cross sectionsare so large that the shape of the rail is without any substantialinfluence. Furthermore, it has been tried to counteract the heatstresses by subdividing the preburned carbon bottom into a plurality ofblocks and thereby to increase the amount of joint filling carbonaceousmass having a greater extensibility.

By electric heating of the iron rails, maximum expansion of the same wastried to be caused prior to the coking of the joint filling mass inorder to prevent in this manner a mechanical destruction of the carbonbody. The composition of the joint filling carbonaceous masses as wellas of the preburned carbon bodies was also subject of special attention.Finally, it was proposed to discontinue the iron rails in the center ofthe carbon blocks or to try to introduce current only through laterallyarranged nipples although by preceeding in this manner the voltagelosses were increased and the current distribution in the cathode becameunfavorable.

However, all of these efforts were not fully successful because thebasic difficulty which was caused by the difference in the heatexpansion of the current conducting iron rail or the like on the onehand and the carbon block on the other hand could not be avoided.Furthermore, the difficulty arose that the temperature gradient withinthe cathode caused in addition to longitudinal and transverse expansionof the iron rails also an upward arching of the same. Thus, the by farlargest number of cathodes is already mechanically damaged prior tostarting operation of the cell by the formation of more or less fineexpansion cracks in the joints between the individual preburned carbonbodies or even within the individual preburned carbon bodies. Duringoperation of the cell, these cracks will be filled with solidifiedmolten electrolyte and thereby the effectiveness of the cell will bereduced. If it happens that molten electrolyte or aluminum reaches theinsulating material beneath the cathode, then the reactions of thevarious materials will cause increase in volume thereof which willsupport the destructive thermal forces.

It is therefore an object of the present invention to overcome the abovediscussed difficulties and disadvantages in the arrangements for currentsupply to carbon cathodes of electrolytic cells of the type described.

It is another object of the present invention to provide in a simple andeconomical manner a current supply for the carbonaceous cathodes of suchelectrolytic cells substantially without causing heat stresses and thedamage caused by the same.

Other objects and advantages of the present invention will becomeapparent from a further reading of the decription and of the appendedclaims.

With the above and other objects in view, the present inventioncontemplates in an electrolytic cell for recovery of a metal from amolten electrolyte containing the same, in combination, a vessel adaptedto hold molten electrolyte the vessel having a bottom formed of asubstantially horizontally extending carbonaceous cathode plate havingan upper face adapted to contact the molten electrolyte and a lowerface, a plurality of current conducting nipples having upper and lowerportions and extending spaced from each other with their upper portionsinto the cathode plate through said lower face thereof, and currentsupply means operatively connected to the lower portions, respectively,of the nipples spaced from the cathode plate.

Thus, the difficulties and disadvantages in the prior art structure ofelectrolytic cells for the recovery of metals from molten electrolyte,particularly for the recovery of aluminum are avoided according to thepresent invention by providing a plurality of nipples which extend,spaced from each other, upwardly into the carbon cathode, whereby thecurrent supply means such as bus bars or iron rails which supply currentto the nipples are located outside of and spaced from the cathode. Inthis manner it is achieved that current will be conducted with verylittle resistance from the nipple into the carbon cathode, due to thegreater heat expansion of the nipple and the consequent firm contactbetween the upper nipple portion and the surrounding portion of thecarbon cathode. On the other hand, in view of the relatively smalldiameter of the nipple, this pressure exerted by the nipple against thesurrounding portion of the carbon cathode will not suffice formechanical destruction or damage of the carbon cathode, particularly ifthe nipple is of circular cross section so that a notch effect will beavoided. Due to arrangement of the bus bars spaced from the highlyheated cathode, the heat expansion which will cause upward arching ofthe bus bars is considerably less than in prior art structures. Thereby,the damage to the cathode during the first heating of the same isavoided and increase in the volume of the cathode by penetration ofcracks therein with molten electrolyte or metal will not take place. Theend result is a considerably prolonged useful life span of the cell.

The nipples preferably extend perpendicular to the horizontallyextending carbon cathode, i.e. in vertical direction from the lower faceof the carbon cathode towards the molten electrolyte on top thereof.However, in most cases it will be preferred to have the nipplesterminate within the center portion of the cathode and not to extend tothe upper face thereof so that contact between molten electrolyte andthe nipple will be avoided. In order to further counteract any heatstresses which still might occur, it is provided according to thepresent invention that the space which separates the cathode from thebus bars is formed as a cooling channel or is filled with heatinsulating material. Aluminum oxide has been found to be particularlysuitable as such heat insulating material which can be blown into thespace or cooling channel between the lower face of the cathode and thebus bars.

The nipples may be made of iron or iron alloys or other electricallyhighly conductive materials, which, however, must have a high meltingpoint, higher than l,100 C. Non-iron materials which may be successfullyused for forming the nipples thereof include titanium borides andcarbides.

The bus bars may be made of iron or iron alloys or of other materialswhich possess an even better electric conductivity and which may have alower melting point than iron. Since according to the present inventionthe bus bars are located outside the cathode and are not in contact withthe same and are not subjected to such very high heat stresses, it ispossible to form the bus bars for instance of copper or of aluminumalloys of suitable composition, for instance of iron-containing aluminumalloys, providing that the material of which the bus bars are formed hasa melting point which is at least as high as about 600 C.

It is further provided according to the present invention that thenipples are surrounded in the cathode with material which is stamped orpoured around the nipples, for instance a carbonaceous mass which iscapable of coking may be used to fill the space between the nipple andthe surrounding cathode portion, or a metal such as iron may be pouredinto this space.

As a stamping mass, i.e. as a carbonaceous mass which can be used toform a firm contact between the nipple and the surrounding portion ofthe carbon cathode may be used a mixture of about 30-35% pitch coke,between 45 and 50% broken coke and between 25 and pitch.

As pointed out further above, and as illustrated in the drawing, it isadvantageous to terminate the cathode nipples, particularly if the sameare formed of iron, within the carbon blocks and spaced from the upperface of the carbon cathode so that contact between liquid aluminum andiron nipple portions and dissolution of the iron nipple portions will beavoided. However, if the nipples are formed of material which isresistant to liquid aluminum, such as titanium carbides or titaniumborides, then the nipples may extend throughout the entire thickness ofthe carbon cathode into direct contact with the aluminum at the upperface of the carbon cathode since thereby the conductivity and thepassage of current through the cathode into the molten electrolyte willbe improved.

The bus bars which supply current to the nipples may extend horizontallyor vertically downward. Thus, the bus bars may extend through the sidewall or through the bottom wall of the outer shell of the cell.

It is of particular advantage if the current conducting members for eachnipple are at least partially flexible. Either the entire currentconducting element may be flexible, for instance in the shape of bands,strips or the like, or a rigid bus bar may be used as the main portionof the current conductive element and only the contact between this busbar and the individual nipples is then provided by a flexible metalband, strip or the like.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings, inwhich:

FIG. 1 is a fragmentary vertical cross section of a cell according tothe present invention showing a rigid bus bar and a flexible metal stripconnection between the rigid bus bar and the individual nipples;

FIG. 2 is a fragmentary elevational cross sectional view of anotherembodiment of the present invention according to which the entirecurrent conducting element for each nipple consists of a flexible stripwhich extends horizontally through the outer lateral shell of the cell.

FIG. 3 is a fragmentary elevational cross sectional view somewhatsimilar to that of FIG. 2, however, showing individual currentconducting elements which extend vertically downwardly through the outerbottom of the cell.

FIG, 4 is a transverse sectional plan view taken in the plane whichincludes the lower faces of the bus bars along line 44 in FIG. 1 in thedirection of the arrows and illustrating the entire cross section of thecell;

FIG. 5 is a transverse sectional elevation of the structure shown inFIG. 3 showing the entire cross section of the cell;

FIG. 6 shows the entire cross section of the cell of which a part isshown in FIG. 2; and

FIG. 7 is an enlargement of part of the structure shown in FIG. 2.

Referring now to the drawings, it will be seen that cathode 1 extendshorizontally along the bottom of the respective cell and is formed inconventional manner of a carbonaceous mass by stamping, pouring andsubsequent burning or of preburned carbon blocks. The nipples areindicated by reference numeral 2 and it is apparent from the drawingthat a considerable number of nipples of relative small diameter arearranged spaced from each other whereby the distance between theindividual nipples will be so chosen as to achieve any desired currentdensity at the corresponding portion of the cathode. The currentconducting members 3 are located outside of and spaced from cathode 1.Between the current conducting members 3 and cathode 1 a free space 4 isformed which may be either used as a cooling channel or which may befilled with heat insulating material, and aluminum oxide has been foundparticularly suitable as a heat insulating material which may be blowninto space 4. Nipples 2 may be formed of iron or iron alloys or otherelectrically highly conductive materials which possess a sufficientlyhigh melting point, preferably higher than l,100 C. The currentconducting members 3 may be formed of iron or iron alloys or of otherand even more highly conductive materials which have a lower meltingpoint. It is possible to form current conducting members 3 of materialshaving a lower melting point down to about 600 C. because of the currentconducting members 3 are located outside of the cathode and thus willnot be exposed to the higher temperatures prevailing in the cathode,

Nipples 2 are stamped into cathode 1 or surrounded by a poured materialindicated by reference numeral 5. The surrounding layer 5 may consisteither of a carbonaceous mass for instance of the composition describedabove which is stamped about the nipples, or may be a solidified pouredmetal having a sufiiciently high melting point, for instance iron.

The only difference between the embodiments of FIG. 2 and FIG. 3 will befound in the manner in which the current conducting members 3 arearranged within the outer shell of the electrolytic cell. According toFIG. 2, the current conducting members extend horizontally and passthrough the side wall of the shell 6, while according to FIG. 3 thecurrent conducting members extend vertically downwardly and pass throughthe bottom of shell 6.

The current conducting elements are either completely or partially offlexible construction. As shown in FIG. 1 the major portion of thecurrent conducting element consists of a rigid bus bar and a flexiblestrip or band connects the bus bar with the respective nipple. Accordingto FIGS. 2, 3 and 7, the entire current conducting member 3 is formed offlexible bands, strips or the like. The flexible portions of the currentconducting members 3 serve for evening out the heat stresses since theflexible portions are capable of absorbing the occurring heat expansionsof the current conducting members. It is immediately apparent thatwhatever heat expansion of the current supplying members 3 including busbars 3a will occur, will be without any effect on carbon cathode 1.

As illustrated, nipples 2 terminate within the interior of cathode 1,about in the center portion of the same or even somewhat below. However,as pointed out above, it is also possible to extend the nipplesthroughout the entire thickness of the cathode so that the same willcome into contact with molten aluminum at the upper face of the cathode,provided that the nipples are formed of a material which will not beaffected by the molten aluminum.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofelectrolytic cells differing from the types described above.

While the invention has been illustrated and described as embodied in anelectrolytic cell for the production of aluminum from moltenelectrolyte, it is not intended to be limited to the details shown,since various modifications and structural changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. In an electrolytic cell for recovery of a metal from a moltenelectrolyte containing the same, in combination, a vessel adapted tohold molten electrolyte, said vessel having a bottom formed of asubstantially horizontally extending carbonaceous cathode plate havingan upper face adapted to contact the molten electrolyte or molten metalelectrolytically separated therefrom, and a lower face; a plurality ofcurrent conducting nipples extending spaced from each other insubstantially vertical direction through said lower face of said cathodeplate into the latter, each of said nipples having a lower end and anupper free end downwardly spaced from said upper face of said cathodeplate and completely separated from said top face by an uninterruptedportion of said cathode plate so as to be out of direct contact withsaid electrolyte; and current supply means spaced from said cathodeplate and electrically connected to said lower end of each nipple.

2. In an electrolytic cell as defined in claim 1, said vessel having afree space beneath said cathode plate, and said current supply meansextending into said space in engagement with said nipples but spacedfrom said cathode plate.

3. In an electrolytic cell as defined in claim 1, said vessel having afree space beneath said cathode plate, and said current supply meansextending into said space in engagement with said nipples but spacedfrom said cathode plate; and a heat insulating material substantiallyfilling said free space.

4. In an electrolytic cell as defined in claim 1, said vessel having afree space beneath said cathode plate, and said current supply meansextending into said space in engagement with said nipples but spacedfrom said cathode plate; and aluminum oxide as a heat insulatingmaterial substantially filling said free space.

5. In electrolytic cell as defined in claim 1, wherein said plurality ofcurrent conducting nipples are formed of an electrically conductivematerial having a melting point of at least 1100 C.

6. In electrolytic cell as defined in claim 1, wherein said currentsupply means have a melting point of at least 600 C.

7. In electrolytic cell as defined in claim 1, and including a currentconducting binder material interposed between and firmly connecting saidnipples, respectively, and said cathode plate.

8. In electrolytic cell as defined in claim 1, wherein said plurality ofcurrent conductive nipples are formed of an electrically conductivematerial selected from the group consisting of iron, titanium carbideand titanium boride.

9. In an electrolytic cell for recovery of a metal from a moltenelectrolyte containing the same, in combination, a vessel adapted tohold molten electrolyte said vessel having a bottom formed of asubstantially horizontally extending carbonaceous cathode plate havingan upper face adapted to contact the molten electrolyte or molten metalelectrolytically separated therefrom, and a lower face; a plurality ofcurrent conducting nipples having upper and lower portions andextending, without contacting said molten electrolyte in substantiallyvertical direction spaced from each other and from the upper face ofsaid cathode plate, with their upper portions into the interior of saidcathode plate through said lower face thereof with portions of saidcathode plate interposed between said nipples and the molten electrolyteor molten metal a current conducting binder material interposed betweensaid firmly connecting said nipples, respectively, and said cathodeplate; and current supply means operatively connected to said lowerportions, respectively, of said nipples spaced from said cathode platesaid current supply means including substantially horizontally extendingbus bars and resilient conductors connecting said bus bars to the lowerportions of said nipples, respectively.

10. In an electrolytic cell for recovery of a metal from a moltenelectrolyte containing the same, in combination, a vessel adapted tohold molten electrolyte said vessel having a bottom formed of asubstantially horizontally ex tending carbonaceous cathode plate havingan upper face adapted to contact the molten electrolyte or molten metalelectrolytically separated therefrom, and a lower face; a

plurality of current conducting nipples having upper and lower portionsand extending, without contacting said molten electrolyte insubstantially vertical direction spaced from each other and from theupper face of said cathode plate, with their upper portions into theinterior of said cathode plate through said lower face thereof withportions of said cathode plate interposed between said nipples and themolten electrolyte or molten metal a current conducting binder materialinterposed between and firmly connecting said nipples, respectively, andsaid cathode plate; and current supply means operatively connected tosaid lower portions, respectively, of said nipples spaced from saidcathode plate said current supply means including substantiallyvertically expanded bus bars and resilient conductors connecting saidbus bars to the lower portions of said nipples, respectively.

8 References Cited UNITED STATES PATENTS 5/1930 Westly 204--280 X 6/1960Allen 204243 4/ 1962 Ransley 204243 4/1965 Ramsey 13-25 FOREIGN PATENTS3/1961 Germany.

JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S.Cl. X.R.

1. IN AN ELECTROLYTIC CELL FOR RECOVERY OF A METAL FROM A MOLTENELECTOLYTE CONTAINING THE SAME, IN COMBINATION, A VESSEL ADAPTED TO HOLDMOLTEN ELECTROLYTE, SAID VESSEL HAVING A BOTTOM FORMED OF ASUBSTANTIALLY HORIZONTALLY EXTENDING CARBONACEOUS CATHODE PLATE HAVINGAN UPPER FACE ADAPTED TO CONTACT THE MOLTEN ELECTROLYTE OR MOLTEN METALELECTROLYTICALLY SEPARATED THEREFROM, AND A LOWER FACE; A PLURALITY OFCURRENT CONDUCTING NIPPLES EXTENDING SPACED FROM EACH OTHER INSUBSTANTIALLY VERTICAL DIRECTION THROUGH SAID LOWER FACE OF SAID CATHODEPLATE INTO THE LAT-